08/28/2012

Observing Highlights for September: The Harvest Moon and Opposition of Uranus

In early August, Mars, Saturn and Spica formed a triangle.
Credit: John
Chumack

The Harvest Moon, which is defined as the Full Moon closest
to the autumnal equinox, can occur in September or October. Most years the
Harvest Moon is in September, as it is in 2012. The equinox, and therefore the
first day of fall, lands on September 22 at 7:49
a.m. Pacific Time. With days and nights of equal
length, the sun rises directly in the east and sets directly in the west. The
closest Full Moon to this date is on September 29. The Harvest Moon rises in
the east just before it hits 100 percent illumination at 8:19
p.m. pacific.

Moonrise in September occurs as little as a half hour apart
from night to night as opposed to other times of the year when moonrise from
one night to the next can fall more than an hour apart. The full and nearly
full moons after sunset in September provided the extra light for farmers in
the fields and earned this moon the name of Harvest Moon.

The planet Uranus reaches opposition, or opposite the sun in
the sky when it rises at sunset and sets at sunrise, on the same night as the
Full Moon, September 29. Normally opposition is considered the best time to
observe a planet, but when it occurs simultaneously with a Full Moon in the
same region of sky, this is not the case. The moon will be just four degrees
above Uranus, washing out the sky around the dim planet.

Saturn and Mars are still keeping close quarters in the west
after sunset, but Saturn sinks toward the horizon as Mars speeds away. By
September 4, Mars is already leaving the constellation Virgo as it crosses the
border into Libra.

The constellation Libra is home to two stars with the
fun-to-pronounce names of Zubenelgenubi and Zubeneschamali, or the southern and
northern claw. Libra was once part of the constellation Scorpius the Scorpion,
thus the claw reference. On September 14, Mars will be just below Zubenelgenubi,
the lower of the two stars that make up the triangular roof-shape of Libra
(which looks a bit like a ramshackle house). Through binoculars, you’ll see
that Zubenelgenubi is a double star system, with the brighter component at
magnitude 2.7 and the dimmer at 5.1.

On September 17, a crescent moon passes below Spica and
Saturn and then skims about two degrees left of Mars on September 19. Mars will
continue heading east across the constellations, entering the Milky Way in
October.

With the onset of autumn, the season’s constellations will
be rising in the east in the evening. The hallmarks of fall include Pegasus
with its Great Square,
Andromeda with its famous galaxy, Perseus with the Double Cluster, and Auriga
with the sparkling star Capella.

For those up late on September evenings, look for the
Pleiades cluster rising with the planet Jupiter just behind. Late at night on
September 7, a half moon lies less than three degrees from Jupiter. The
orangish star Aldebaran is to the right of the moon along with the Hyades
cluster.

Kelly Kizer Whitt loves clean, clear, and dark skies. Kelly studied English and Astronomy at the University of Wisconsin-Madison and worked for Astronomy magazine. She writes the SkyGuide for AstronomyToday.com. You can follow her on Twitter at twitter.com/Astronomommy.

User comments or postings reflect the opinions of the responsible contributor only, and do not reflect the viewpoint of the Sierra Club. The Sierra Club does not endorse or guarantee the accuracy of any posting. The Sierra Club accepts no obligation to review every posting, but reserves the right (but not the obligation) to delete postings that may be considered offensive, illegal or inappropriate.